Energy, Year 8: more practice

600

HintEnergy transferred is power multiplied by time.

The why60 W is 60 J every second, and 60 × 10 = 600 J.

The hairdryer

HintWatts measure how quickly energy is transferred.

The whyA higher power rating means more joules every second, so the hairdryer transfers energy about 180 times as quickly.

6

HintEnergy in kWh = power in kW × time in hours.

The why3 kW × 2 h = 6 kWh.

180

HintCost = number of units × price per unit.

The why6 × 30 = 180 p, which is £1.80.

The metal conducts energy away from your hand faster

HintThink about how fast heat leaves skin.

The whyMetals are good conductors, so they carry energy away from your warm fingers quickly. Wood is an insulator, so it does not.

Its particles cannot flow

HintWhat has to move for convection to carry energy?

The whyConvection needs a fluid whose particles can flow and carry energy with them. In a solid the particles only vibrate in place.

By radiation

HintNothing is there to be heated and passed along.

The whyRadiation does not need particles to travel through, unlike conduction and convection.

Infrared radiation

HintIt is below red in the spectrum.

The whyAll objects emit infrared. The hotter an object is, the more it emits.

White surfaces reflect most of the radiation

HintThink of what a light, shiny surface does with the Sun's infrared.

The whyWhite surfaces are poor absorbers, so less radiation is absorbed and the inside stays cooler.

It reflects the person's own infrared back

HintThink about what a shiny surface does with radiation.

The whyA shiny surface is a poor emitter and a good reflector of infrared, so energy radiated by the body is reflected back rather than lost.